The designer of a blown-film extrusion die needs to bear in mind the following requirements:
1. the plastic film should be of high quality--which means that the surface of the sheet should be smooth and transparent; PA1 2. production should be done economically--which means that the velocity with which the plastic melt emerges from the annular nozzle should be maximised.
These two requirements are in conflict, in that as the melt-emergence velocity is increased, so the likelihood of surface-fracture is increased. Surface-fracture is a quality fault, in which the surface of the plastic film becomes crazed, or fogged.
It is known that the onset of surface-fracture can be deferred by increasing the temperature of the plastic melt. However, increasing the temperature of the melt, as it emerges from the nozzle, introduces further difficulties, and it is known that the better approach is to increase the temperature of just the surface of the melt, while leaving the bulk of the melt at a cooler temperature. This is done conventionally by means of heaters installed on the lips of the nozzle, i.e by so-called die-lip heaters.
If the bulk of the plastic melt were too hot, the emerging bubble would be too liquid (i.e too runny), whereby the bubble would become difficult to control. The cooler the melt, the more the melt has some degree of ability to support itself, and the more the bubble tends to behave in a predictable and controllable manner.
The viscosity of hot melt is very sensitive to temperature. Adding a few degrees to the temperature of just the surface of the melt, while leaving the main flow of the melt relatively cool, means that the surface can be considerably less viscous (i.e more liquid) than the main flow. It has been suggested that die-lip heaters are effective because the increased liquid-ness of the surface of the melt lubricates the die-lips, thereby easing the melt surface through the nozzle, and thereby deferring the onset of surface-fracture.
The invention is aimed at providing an improved system for heating the die-lips.
The invention is aimed at providing a manner of mounting the heating element, in which heat from the element is transferred to the die-lip evenly all around the circumference of the die-lip, and in which the rest of the bulk of the die is insulated therefrom.
The annular nozzle includes, of course, both an inner lip and an outer lip. Usually (though not always) heaters would be fitted on both die-lips.